Radiolucent spinal fusion cage

ABSTRACT

An improved bone graft is provided for human implantation, particularly such as a spinal fusion cage for implantation into the inter-vertebral space between two adjacent vertebrae. The improved spinal fusion cage includes a substrate block of high strength biocompatible material having a selected size and shape to fit the anatomical space, and a controlled porosity analogous to natural bone. The substrate block may be coated with a bio-active surface coating material such as hydroxyapatite or a calcium phosphate to promote bone in growth and enhanced bone fusion. Upon implantation, the fusion cage provides a spacer element having a desired combination of mechanical strength together with osteoconductivity and osteoinductivity to promote bone ingrowth and fusion, as well as radiolucency for facilitated post-operative monitoring. The fusion cage may additionally carry one or more natural or synthetic therapeutic agents for further promoting bone ingrowth and fusion.

This is a continuation-in-part of U.S. Ser. No. 10/137,108, filed Apr.30, 2002, which in turn claims the benefit of U.S. ProvisionalApplication No. 60/287,824, filed May 1, 2001.

BACKGROUND OF THE INVENTION

This invention relates generally to improvements in bone grafts such asspinal fusion cages of the type designed for human implantation betweenadjacent spinal vertebrae, to maintain the vertebrae in substantiallyfixed spaced relation while promoting interbody bone ingrowth and fusiontherebetween. More particularly, this invention relates to animplantable bone graft such as a spinal fusion cage having an improvedcombination of enhanced mechanical strength together with osteoinductiveand osteoconductive properties, in a device that additionally andbeneficially provides visualization of bone growth for facilitatedpost-operative monitoring.

Implantable interbody bone grafts such as spinal fusion devices areknown in the art and are routinely used by spine surgeons to keepadjacent vertebrae in a desired spaced-apart relation while interbodybone ingrowth and fusion takes place. Such spinal fusion devices arealso used to provide weight bearing support between adjacent vertebralbodies and thus correct clinical problems. Such spinal fusion devicesare indicated for medical treatment of degenerative disc disease,discogenic low back pain and spondylolisthesis. These conditions havebeen treated by using constructs, typically made from metals such astitanium or cobalt chrome alloys such as used in orthopedic implants,and allograft (donor) or autograft (patient) bone to promote boneingrowth and fusion.

Typical interbody spinal fusion devices, such as plugs for example, havehollow or open spaces that are usually filled with bone graft material,either autogenous bone material provided by the patient or allogenousbone material provided by a third party donor. These devices also havelateral slots or openings which are primarily used to promote ingrowthof blood supply and grow active and live bone. These implants may alsohave a patterned exterior surface such as a ribbed or serrated surfaceor a screw thread to achieve enhanced mechanical interlock betweenadjacent vertebrae, with minimal risk of implant dislodgement from thesite. See, for example, U.S. Pat. Nos. 5,785,710; and 5,702,453. Typicalmaterials of construction for such interbody spinal fusion devicesinclude bio-compatible carbon fiber reinforced polymers, cobalt chromealloys, and stainless steels or titanium alloys. See, for example, U.S.Pat. No. 5,425,772.

Most state-of-the-art spinal fusion implants are made from titaniumalloy and allograft (donor) bone, and have enjoyed clinical success aswell as rapid and widespread use due to improved patient outcomes.However, traditional titanium-based implant devices exhibit poorradiolucency characteristics, presenting difficulties in post-operativemonitoring and evaluation of the fusion process due to the radio-shadowproduced by the non-lucent metal. There is also clinical evidence ofbone subsidence and collapse which is believed to be attributable tomechanical incompatibility between natural bone and the metal implantmaterial. Moreover, traditional titanium-based implant devices areprimarily load bearing but are not osteoconductive, i.e., not conduciveto direct and strong mechanical attachment to patient bone tissue,leading to potential graft necrosis, poor fusion and stability. Bycontrast, allograft bone implants exhibit good osteoconductiveproperties, but can subside over time as they assimilate into naturalbone. Further, they suffer from poor pull out strength resulting in poorstability, primarily due to the limited options in machining the contactsurfaces. Allograft bone implants also have variable materialsproperties and, perhaps most important of all, are in very limitedsupply. A small but finite risk of disease transmission with allograftbone is a factor as well. In response to these problems some developersare attempting to use porous tantalum-based metal constructs, but thesehave met with limited success owing to the poor elastic modulii ofporous metals.

A typical titanium alloy spinal fusion device is constructed from ahollow cylindrical and externally threaded metal cage-like constructwith fenestrations that allow communication of the cancellous hosttissue with the hollow core, which is packed with morselized bone graftmaterial. This design, constrained by the materials properties oftitanium alloys, relies on bony ingrowth into the fenestrations inducedby the bone graft material. However, the titanium-based structure canform a thin fibrous layer at the bone/metal interface, which degradesbone attachment to the metal. In addition, the hollow core into whichthe graft material is packed may have sub-optimal stress transmissionand vascularization, thus eventually leading to failure to incorporatethe graft. Mechanical stability, transmission of fluid stress, and thepresence of osteoinductive agents are required to stimulate the ingrowthof vascular buds and proliferate mesenchymal cells from the cancelloushost tissue into the graft material. However, most titanium-based spinalfusion devices in use today have end caps or lateral solid walls toprevent egress of the graft outwardly from the core and ingress ofremnant disc tissue and fibroblasts into the core.

Autologous (patient) bone fusion has been used in the past and has atheoretically ideal mix of osteoconductive and osteoinductiveproperties. However, supply of autologous bone material is limited andsignificant complications are known to occur from bone harvesting.Moreover, the costs associated with harvesting autograft bone materialare high, requiring two separate incisions, with the patient having toundergo more pain and recuperation due to the harvesting andimplantation processes. Additionally, autologous cancellous bonematerial has inadequate mechanical strength to support intervertebralforces by itself, whereby the bone material is normally incorporatedwith a metal-based construct.

Ceramic materials provide potential alternative structures for use inspinal fusion implant devices. In this regard, monolithic ceramicconstructs have been proposed, formed from conventional materials suchas hydroxyapatitie (HAP) and/or tricalcium phosphate (TCP). See, forexample, U.S. Pat. No. 6,037,519. However, while these ceramic materialsmay provide satisfactory osteoconductive and osteoinductive properties,they have not provided the mechanical strength necessary for theimplant.

Thus, a significant need exists for further improvements in and to thedesign of bone grafts such as spinal fusion implant devices,particularly to provide a high strength implant having high boneingrowth and fusion characteristics, together with substantialradiolucency for effective and facilitated post-operative monitoring.

Hence, it is an object of the present invention to provide an improvedbone graft such as an interbody spinal fusion implant or cage made froma bio-compatible open pore structure, which has a radiolucency similarto that of the surrounding bone. It is also an object of the presentinvention to provide a substrate of high bio-mechanical strength forcarrying biological agents which promote intervertebral bone ingrowth,healing and fusion. It is a further objective of the present inventionto provide an interbody fusion device which has mechanical propertiesthat substantially match that of natural bone.

SUMMARY OF THE INVENTION

In accordance with the invention, an improved bone graft such as aspinal fusion cage is provided for human implantation into the spacebetween a pair of adjacent vertebrae, following removal of disc materialbetween endplates of the adjacent vertebrae, to maintain the adjacentvertebrae in a predetermined and substantially fixed spaced relationwhile promoting interbody bone ingrowth and fusion. In this regard, theimproved spinal fusion cage of the present invention is designed for usein addressing clinical problems indicated by medical treatment ofdegenerative disc disease, discogenic lower back pain, andspondylolisthesis.

The improved bone graft, as embodied in the form of the improved spinalfusion cage, comprises a substrate block formed from a bio-compatiblematerial composition having a relatively high bio-mechanical strengthand load bearing capacity. This substrate may be porous, open-celled, ordense solid. A preferred composition of the high strength substrateblock comprises a silicon nitride ceramic material. The substrate blockmay be porous, having a porosity of about 10% to about 80% by volumewith open pores distributed throughout and a pore size range of fromabout 5 to about 500 microns. When the substrate is porous, the porosityof the substrate block is gradated from a first relatively low porosityregion emulating or mimicking the porosity of cortical bone to a secondrelatively higher porosity region emulating or mimicking the porosity ofcancellous bone. In a second embodiment, the substrate block is a densesolid comprised of a ceramic, metal or polymer material. This densesolid substrate would then be attached to a second highly porous regionemulating or mimicking the porosity of cancellous bone. Preferably, theporous region would be formed around the substrate.

In the method where a dense, solid material is used as the substrateblock, the block will be externally coated with a bio-active surfacecoating material selected for relatively high osteoconductive andosteoinductive properties, such as a hydroxyapatite or a calciumphosphate material. The porous portion is internally and externallycoated with a bio-active surface coating material selected forrelatively high osteoconductive and osteoinductive properties, such as ahydroxyapatite or a calcium phosphate material. The porous region,however, may be in and of itself a bio-active material selected forrelatively high osteoconductive and osteoinductive properties, such as ahydroxyapatite or a calcium phosphate material.

The thus-formed bone graft can be made in a variety of shapes and sizesto suit different specific implantation requirements. Preferred shapesinclude a generally rectangular block with a tapered or lordotic crosssection to suit the required curvature of the inter-vertebral space, inthe case of a spinal fusion device. The exterior superior and inferiorsurfaces of the rectangular body may include ridges or teeth forfacilitated engagement with the adjacent vertebrae. Alternativepreferred shapes include a generally oblong, rectangular block which mayalso include serrations or the like on one or more exterior facesthereof, and/or may have a tapered or lordotic cross section forimproved fit into the inter-vertebral space. A further preferred shapemay include a crescent shape block which may also include serrations orthe like on one or more exterior faces thereof, and/or may have atapered or lordotic cross section for improved fit into theinter-vertebral space. The bone graft may desirably include notches forreleasable engagement with a suitable insertion tool. In addition, thebone graft may also include one or more laterally open recesses or boresfor receiving and supporting osteoconductive bone graft material, suchas allograft (donor) or autograft (patient) material.

Further alternative bone graft configurations may include a densesubstrate region substantially emulating cortical bone, to define a highstrength loading bearing zone or strut for absorbing impaction andinsertion load, in combination with one or more relatively high porositysecond regions substantially emulating cancellous bone for contactingadjacent patient bone for enhanced bone ingrowth and fusion.

The resultant bone graft exhibits relatively high mechanical strengthfor load bearing support, for example, between adjacent vertebrae in thecase of a spinal fusion cage, while additionally and desirably providinghigh osteoconductive and osteoinductive properties to achieve enhancedbone ingrowth and interbody fusion. Importantly, these desirablecharacteristics are achieved in a structure which is substantiallyradiolucent so that the implant does not interfere with post-operativeradiographic monitoring of the fusion process.

In accordance with a further aspect of the invention, the bone graft mayadditionally carry one or more therapeutic agents for achieving furtherenhanced bone fusion and ingrowth. Such therapeutic agents may includenatural or synthetic therapeutic agents such as bone morphogenicproteins (BMPs), growth factors, bone marrow aspirate, stem cells,progenitor cells, antibiotics, or other osteoconductive, osteoinductive,osteogenic, or any other fusion enhancing material or beneficialtherapeutic agent.

Other features and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a perspective view depicting the spinal fusion cage in theinter-vertebral space;

FIG. 2 is a perspective view showing one preferred embodiment of thespinal fusion cage;

FIG. 3 is a perspective view showing the load bearing portion of thedevice of FIG. 2 with anterior and posterior load bearing wallsconnected by a strut, relieved in the superior and inferior aspects;

FIG. 4 is a perspective view depicting one alternative preferred andgenerally rectangular bone graft such as a spinal fusion cage;

FIG. 5 is a perspective view depicting the load bearing portion of thedevice of FIG. 4 with anterior and posterior load bearing wallsconnected by a strut, relieved in the superior and inferior aspects;

FIG. 6 is a perspective view showing still another alternative preferredform of the invention, comprising a generally oblong, rectangular bonegraft such as a spinal fusion cage;

FIG. 7 is a perspective view depicting the load bearing portion of thedevice of FIG. 6 with anterior and posterior load bearing wallsconnected by a strut, relieved in the superior and inferior aspects;

FIG. 8 is an axial view of still another alternative form of theinvention, taken generally on the load bearing axis of the spine,comprising a generally crescent shaped device conforming to the naturalvertebral body shape;

FIG. 9 is a perspective view of the device of FIG. 8, showing a porousposterior margin;

FIG. 10 is a perspective view of the load bearing portion of the deviceof FIG. 8, showing a anterior and lateral load bearing walls connectedby a central strut, relieved in the superior and inferior aspects;

FIG. 11 is an axial view of a further preferred alternative embodimentof the invention, comprising of a generally rectangular shape withmacro-pores;

FIG. 12 is a perspective view of the device of FIG. 11 showing theinterconnection of the macro-pores; and

FIG. 13 is a sectional view of the device of FIG. 11 taken generallyalong the mid-transverse plane 6-6 of FIG. 11 of the device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the exemplary drawings, a radiolucent bone graft such as aspinal fusion cage referred to generally in FIGS. 1-3 by the referencenumeral 10 is provided for seated implantation between a pair ofadjacent patient bones such as spinal vertebrae 12 (FIG. 1) to maintainthe vertebrae in spaced relation while promoting interbody bone ingrowthand fusion. In general, the improved bone graft 10 comprises abio-compatible substrate having a porous construction to define an openlattice conducive to interbody bone ingrowth and fusion, while providinga strong mechanical load bearing structure analogous to the load bearingproperties of cortical and cancellous bone. This open-celled substrateis coated internally and externally with a bio-active surface coatingselected for relatively strong osteoconductive and osteoinductiveproperties, whereby the coated substrate provides a scaffold conduciveto cell attachment and proliferation to promote interbody bone ingrowthand fusion attachment. The substrate may also carry one or more selectedtherapeutic agents suitable for bone repair, augmentation and otherorthopedic uses.

FIGS. 1-3 illustrate the improved bone graft in the form of an improvedspinal fusion cage 10 in accordance with one preferred embodiment, inthe shape of a generally rectangular body having ridges formed on theexposed top and bottom ends or faces 14. The lateral, anterior, andposterior walls of the body having notches 18 for the releasableengagement with an insertion tool.

The preferred substrate composition comprises a relatively high strengthblock 16 (FIG. 3). In accordance with one preferred form of theinvention, this substrate block comprises a relatively dense 16 siliconnitride composition having a controlled porosity and having a suitablesize and shape for seated implantation, such as into the inter-vertebralspace in the case of the spinal fusion cage 10. In a preferred form, theremainder of the substrate is comprised of a relatively porous siliconnitride 20 (FIG. 2) having an open-celled controlled porosity. Onepreferred silicon nitride ceramic material comprises a doped siliconnitride of the type disclosed in copending U.S. Ser. No. 10/171,376,which is incorporated by reference herein.

Moreover, in the preferred form, the pores are arranged with a variableporosity gradient to define a first region of relatively low or reducedporosity (less than about 5%) substantially mimicking cortical bonestructure and a second region of relatively large or increased porosity(ranging from about 30% to about 80%) substantially mimicking cancellousbone structure. In one preferred configuration, the outer or externalsurfaces of the reticulated substrate block comprise the first or lowporosity region for improved load bearing capacity, while the interiorsurfaces of the substrate block comprises the second or high porosityregion mimicking cancellous bone for enhance bone ingrowth and fusion.

This high strength substrate block is surface-coated internally andexternally with a bio-active organic or inorganic surface coatingmaterial selected for relatively strong osteoconductive andosteoinductive properties to provide a nutrient rich environment forcellular activity to promote interbody bone ingrowth and fusionattachment. Preferred surface coating materials comprise a resorbablematerial such as hydroxyapatite or a calcium phosphate ceramic.Alternative glassy (amorphous) materials having a relatively richcalcium and phosphate composition may also be used, particularly whereinsuch materials incorporate calcium and phosphate in a ratio similar tonatural bone or hydroxyapatite. Such glassy compositions may comprise apartially or fully amorphous osteoinductive material comprising acomposite of a glass and osteoinductive calcium compound, with acomposition varying from about 100% glass to 100% osteoinductive calciumcompound. The surface coating may also comprise autologous bone marrowaspirates.

The resultant bone graft 10 thus comprises the substrate block formedfrom the high strength material having bio-mimetic properties and whichis nonresorbable, or slowly or infinitely slowly resorbable whenimplanted into the patient, in combination with the bio-active surfacecoating which is comparatively rapidly resorbable to promote rapid andvigorous bone ingrowth activity.

The substrate block may also advantageously be coated or impregnatedwith one or more selected therapeutic agents, for example, such asautologous, synthetic or stem cell derived growth factors or proteinsand growth factors such as bone morphogenic protein (BMP) or a precursorthereto, which further promotes healing, fusion and growth. Alternativetherapeutic agents may also include an antibiotic, or naturaltherapeutic agents such as bone marrow aspirates, and growth factors orprogenitor cells such as mesenchymal stem cells, hematopoietic cells, orembryonic stem cells, either alone or as a combination of differentbeneficial agents.

The resultant illustrative spinal fusion cage 10 exhibits relativelyhigh bio-mechanical strength similar to the load bearing characteristicsof natural bone. In addition, the spinal fusion cage 10 exhibitsrelatively strong osteoconductive and osteoinductive characteristicsattributable primarily to the surface coating, again similar to naturalbone. Importantly, the fusion cage 10 is also substantially radiolucent,so that the fusion cage does not interfere with post-operativeradiological analysis of interbody bone ingrowth and fusion.

The relatively dense, high strength portion 16 is preferably formed in amanner and with exposed faces or ends 14 with which to withstand theaxial loading of the spine. In the preferred embodiment as shown, theanterior and posterior walls of the device are formed as part of thishigh strength portion, each with exposed upper and lower ends or faces14. This is done to allow the high strength region to interface with thecortical ring of the adjacent vertebral body 12. Additionally, a strut22 of the high strength material extends between the anterior andposterior walls, which beneficially provides a load bearing structurecapable of withstanding impaction and insertion loading in theanterior-posterior direction. Consequently, the relatively porousportion is formed in-between the dense anterior-posterior walls andaround the central strut. The porous portion thereby forms the remainderof the device, including a large region of the superior, inferior, andlateral aspects. The porous portion, being less dense in nature than thehigh strength regions of the device, is increasingly radiolucent, thusallowing for assessment of bone growth and bony attachment to theadjacent vertebral body.

FIGS. 4-10 illustrate alternative configurations for improved bonegrafts such as spinal fusion cages constructed in accordance with thepresent invention, it being recognized and understood that the bonegraft can be constructed in a wide range of different geometric sizesand shapes. FIG. 4 shows a spinal fusion cage 110 having a generallyrectangular shape similar to the fusion cage 10 shown and described inFIGS. 1-3, but the form is elongated, as for use in replacing an entirevertebral body. As shown, the spinal fusion cage 110 (FIG. 5) has arelatively dense structure defined by a high strength substrate block112 (as previously described) coated with the bio-active surface coatingmaterial, but wherein the relatively dense interior structure is definedmultiple struts 116 with high strength for withstanding impaction andinsertion loading in an anterior-posterior direction between anteriorand posterior walls with exposes upper and lower ends or faces. Themultiple struts 116 additionally create interior openings which providefor lateral fluid transmission and optimize bone growth laterallythrough the center of the implant. FIG. 5 shows multiple dense struts,thereby demonstrating that the porous region is able to make contactwith the adjacent superior and inferior vertebrae. The porous region 114is more radiolucent than the surrounding dense portion and thereforeprovides enhanced visualization for analysis of bone growth andsubsequent fusion with the adjacent vertebrae. Each of the embodimentsdepicted in FIGS. 1-13 has a height dimension and may be tapered orlordotic in shape for enhanced anatomical fit, for example, into theinter-vertebral space or the like.

FIGS. 6-7 depicts still another alternative preferred embodiment of agenerally oblong, rectangular geometry 410 having both a high strength,dense region 40, as well as a relatively porous region 44 for bonein-growth. This geometry would be useful for surgical approaches inwhich it is necessary to place two implants next to each other in theintervertebral space. More particularly, FIGS. 6-7 show a generallyoblong, rectangular bone graft such as a spinal fusion cage 410 having atapered height dimension in the anterior-posterior direction. Thesubstrate block is formed with the first region 40 of relatively lowporosity substantially mimicking cortical bone to extend across theanterior and posterior faces and further to include at least oneinterconnecting load bearing strut 42 shown in the illustrative drawingsto extend centrally in an anterior-posterior direction within the bodyof the substrate block. The remainder of the substrate block comprisesthe second portion 44 of relatively high porosity substantiallymimicking cancellous bone. The harder first region 40 including thecentral strut 42 beneficially provides a hard and strong load bearingstructure analogous to that shown and described with respect to FIGS.1-5, and capable of withstanding impaction and insertion forces in theanterior-posterior direction without damage to the implant, while thesofter second region 44 presents an exposed and large surface area forsubstantially optimized interknitting ingrowth and fusion with adjacentpatient bone. In a spinal fusion cage application, the medial-lateralfaces of the implant are advantageously defined by the softer secondregion 44, wherein these regions are thus exposed to traditionalmedial-lateral X-ray imaging for post-operative radiological analysis ofthe implant/bone interface. Persons skilled in the art will recognizeand appreciate that alternative configurations for the load bearingstrut or struts 42 may be used, such as an X-shaped strut configurationextending in a cranial-caudial direction, in combination with or in lieuof the exterior faces 40 and/or the anterior-posterior central strut asshown.

FIGS. 8-10 depict a further alternative preferred form of the invention,with a generally crescent shaped geometry 510. The substrate block isformed of a relatively dense, high strength region 50 substantiallymimicking cortical bone extending along the anterior and lateral wallsand including exposed upper and lower ends or faces. The dense portion50 once again beneficially provides a strong load bearing structurecapable of withstanding axial loads in the spine. Also, thehigh-strength region 50 is located along the anterior of the substrate,thereby interfacing with the load bearing cortical bone of the adjacentvertebral body. An integral dense strut 52 extends between the denselateral walls providing a load bearing structure for impaction andinsertion forces exhibited in a lateral approach. The superior,inferior, and posterior portions of the substrate are formed with arelatively porous material 54. This provides for bone growth andincreased radiolucency.

FIGS. 11-14 depict a still further alternative preferred embodimentwhich is formed entirely of a relatively low porosity, high-strengthsubstrate 610. The subsequent porous structure 60 is created by drillingor boring a plurality of macro-pores 62 into the superior, inferior, andlateral faces of the device. This method allows the anterior andposterior walls to remain intact and thus be able to withstand theloading of the spinal column. The macro-pores are oriented in both theaxial direction of the spine, as well as between the lateral walls ofthe device, thereby allowing bone to grow in the direction of the spinalloading and laterally through the substrate. The macropores arepositioned in such a manner as to allow for continuous interconnection70, thereby creating a meshwork of pores for bony ingrowth into thedevice. The macropores extend either from one face of the device to theopposite face 64, or towards the center of the device, extended to acertain depth, and terminated therein 66. The blind macropores 66in-turn create a portion in the center of the device which remains solidand is therefore a load bearing strut 68 extending from the anteriorwall to the posterior wall and capable of withstanding impaction andinsertion loads in the anterior-posterior direction. This macroporemethod can also be utilized with geometries similar to those depicted inFIGS. 6-10, such as the oblong rectangular 410 and the crescent 510.

In all of the embodiments of FIGS. 1-13, the substrate block comprises ahigh strength porous ceramic as previously described, and is coated withthe bio-active surface coating material, again as previously described,to enhance bone ingrowth and fusion. The substrate block may alsoinclude one or more therapeutic agents. Persons skilled in the art willrecognize and appreciate that the relatively low and high porosityregions 16 and 20 shown in FIGS. 2-3 will be integrally joined by asuitable albeit relatively narrow gradient region wherein the porositytransitions therebetween.

The improved bone graft such as the illustrative spinal fusion cage ofthe present invention thus comprises an open-celled substrate blockstructure which is coated with a bio-active surface coating, and has thestrength required for the weight bearing capacity required of a fusiondevice. The capability of being infused with the appropriate biologiccoating agent imparts desirable osteoconductive and osteoinductiveproperties to the device for enhanced interbody bone ingrowth andfusion, without detracting from essential load bearing characteristics.The radiolucent characteristics of the improved device beneficiallyaccommodate post-operative radiological examination to monitor the boneingrowth and fusion progress, substantially without undesirableradio-shadowing attributable to the fusion cage. The external serrationsor threads formed on the fusion cage may have a variable depth to enablethe base of the device to contact the cortical bone for optimal weightbearing capacity. In addition to these benefits, the present inventionis easy to manufacture in a cost competitive manner. The invention thusprovides a substantial improvement in addressing clinical problemsindicated for medical treatment of degenerative disc disease, discogeniclow back pain and spondylolisthesis.

The bone graft of the present invention provides at least the followingbenefits over the prior art:

-   -   [a] a porous osteoconductive scaffold for enhanced fusion rates;    -   [b] a bio-mimetic load bearing superstructure providing        appropriate stress transmission without fatigue failure;    -   [c] a pore structure and size suitable for ingrowth and        vascularization,    -   [d] the ability to absorb and retain an osteoinductive agent        such as autologous bone marrow aspirate or BMPs;    -   [e] bio-inert and bio-compatible with adjacent tissue and        selected for ease of resorption;    -   [f] fabricatable and machinable into various shapes;    -   [g] sterilizable; and    -   [h] low manufacturing cost.

A variety of further modifications and improvements in and to the spinalfusion cage of the present invention will be apparent to those personsskilled in the art. In this regard, it will be recognized and understoodthat the bone graft implant can be formed in the size and shape of asmall pellet for suitable packing of multiple implants into a boneregeneration/ingrowth site. Accordingly, no limitation on the inventionis intended by way of the foregoing description and accompanyingdrawings, except as set forth in the appended claims.

1. A spinal fusion cage for implantation between and fusion withadjacent vertebrae, comprising: a substrate block having a first regionof relatively high strength corresponding substantially with naturalcortical bone and a second region of porous form correspondingsubstantially with natural cancellous bone.
 2. The spinal fusion cage ofclaim 1 wherein said substrate block where either the first or secondportion comprises a ceramic structure formed from silicon nitride,alumina, zirconia, zirconia toughened alumina, hydroxyapatite, calciumphosphate, or composition thereof.
 3. The spinal fusion cage of claim 1wherein said substrate block where either the first or second portioncomprises a metallic structure formed from titanium, tantalum, stainlesssteel, cobalt chrome alloy, or composition thereof.
 4. The spinal fusioncage of claim 1 wherein said substrate block where either the first orsecond portion comprises a polymeric structure formed from peek, carbonfiber reinforced polymer, PMMA, PLA (or other bioresorbable polymer), orcomposition thereof.
 5. The spinal fusion cage of claim 1 wherein saidsubstrate block where either the first or second portion comprises aflexible material formed from silicone, polyurethane silicone,hydrogels, elastomers, or composition thereof.
 6. A spinal fusion cageof claim 1 wherein a bio-active and resorbable surface coating appliedto said substrate block, said surface coating having osteoconductive andosteoinductive properties to promote interbody bone ingrowth and fusionattachment with the adjacent vertebrae.
 7. The spinal fusion cage ofclaim 1 wherein said substrate block where the second portion iscomprised of a bio-active and resorbable material having relatively highosteoconductive and osteoinductive properties, such as a hydroxyapatiteor a calcium phosphate material.
 8. The spinal fusion cage of claim 1wherein said substrate block where the first portion being relativelynon-resorbable or resorbable at a rate substantially less than thesecond portion.
 9. The spinal fusion cage of claim 1 wherein the firstregion and the second region of the said substrate block has a porosityranging from about 0% to about 80% by volume, and further wherein thepore size ranges from about 1 micron to about 1,500 microns.
 10. Thespinal fusion cage of claim 9 wherein the said first region of the saidsubstrate block has porosity ranges from about 0% to about 50% byvolume, and wherein the pore sizes range from about 1 micron to about500 microns.
 11. The spinal fusion cage of claim 9 wherein the saidsecond region of the said substrate block has porosity ranges from about30% to about 80% by volume, and wherein the pore sizes range from about100 microns to about 1000 microns.
 12. The spinal fusion cage of claim 9wherein the said substrate block has a variable porosity gradientsubstantially mimicking natural cortical and cancellous bone.
 13. Thespinal fusion cage of claim 9 wherein said substrate block has a firstregion of relatively low porosity substantially mimicking naturalcortical bone, and a second region of relatively high porositysubstantially mimicking cancellous patient bone.
 14. The spinal fusioncage of claim 9 wherein said first region has a porosity of less thanabout 5%, and wherein said second region has a porosity ranging fromabout 30% to about 80%.
 15. The spinal fusion cage of claim 9 whereinsaid first region is generally disposed on the exterior of saidsubstrate block, and said second region is generally disposed on theinterior surfaces of said substrate block.
 16. The spinal fusion cage ofclaim 9 wherein said second region is generally disposed on the exteriorof said substrate block, and said first region is generally disposed onthe interior surfaces of said substrate block.
 17. The spinal fusioncage of claim 13 wherein said first region is generally disposed onanterior and posterior surfaces of said substrate block and furtherdefines at least one structural load bearing strut extending throughsaid substrate block between said anterior and posterior surfaces, saidsecond region including an extended exposed surface area for contactingthe adjacent vertebrae.
 18. The spinal fusion cage of claim 17 whereinsaid second region is substantially exposed on medial and lateralsurfaces of said substrate block.
 19. The spinal fusion cage of claim 13wherein said first region circumferentially surrounds and supports saidsecond region, said second region including an extended exposed surfacearea for contacting the adjacent vertebrae.
 20. The spinal fusion cageof claim 13 wherein said second region circumferentially surrounds saidfirst region, said second region including an extended exposed surfacearea for contacting the adjacent vertebrae.
 21. The spinal fusion cageof claim 13 wherein said first region comprises at least one structuralload bearing strut extending through said substrate block, wherein saidsecond region including an extended exposed surface area for contactingthe adjacent vertebrae.
 22. The spinal fusion cage of claim 1 whereinsaid substrate block further includes means for facilitated grasping andmanipulation with a surgical instrument for implantation.
 23. The spinalfusion cage of claim 6 wherein said bio-active surface coating isinternally and externally applied to said substrate block.
 24. Thespinal fusion cage of claim 6 wherein said bio-active surface coating isselected from the group consisting of hydroxyapatite and calciumcompounds.
 25. The spinal fusion cage of claim 6 wherein said bio-activesurface coating comprises a partially or fully amorphous osteoinductivematerial including a glass and osteoinductive calcium compound.
 26. Thespinal fusion cage of claim 6 wherein said bio-active surface coatingcomprises an organic coating material.
 27. The spinal fusion cage ofclaim 26 wherein said organic coating material is selected from thegroup consisting of autologous bone marrow aspirates, bone morphogenicproteins, growth factors and progenitor cells, and mixtures thereof. 28.The spinal fusion cage of claim 27 wherein said progenitor cells includemesenchymal stem cells, hematopoietic cells, and embryonic stem cells.29. The spinal fusion cage of claim 1 wherein the first region of thesaid substrate block is substantially radiolucent.
 30. The spinal fusioncage of claim 1 wherein the second region of the said substrate block issubstantially radiolucent.
 31. The spinal fusion cage of claim 1 furtherincluding a therapeutic agent carried by said substrate block.
 32. Thespinal fusion cage of claim 31 wherein said therapeutic agent comprisesa natural or synthetic osteoconductive or osteoinductive agent.
 33. Thespinal fusion cage of claim 1 wherein said substrate block has a roughexterior surface.
 34. The spinal fusion cage of claim 1 wherein saidsubstrate block has a ribbed exterior surface.
 35. The spinal fusioncage of claim 1 wherein said substrate block has a laterally open boreformed therein, and further including an osteoconductive materialsupported within said bore.
 36. The spinal fusion cage of claim 35wherein said osteoconductive material comprises morselized bone graftmaterial.
 37. The spinal fusion cage of claim 1 wherein the pores formedwithin the second region of the said substrate block are insubstantially open fluid communication sufficient to transmit fluidpressure therebetween.
 38. The spinal fusion cage of claim 1 wherein thepores formed within the first region of the said substrate block may bein substantially open fluid communication sufficient to transmit fluidpressure therebetween.
 39. A spinal fusion cage for implantation betweenand fusion with adjacent vertebrae, comprising: a substrate block havinga relatively high strength corresponding substantially with naturalcortical and cancellous bone; and a bio-active and relatively rapidlyresorbable surface coating applied to said substrate block, said surfacecoating having osteoconductive and osteoinductive properties to promoteinterbody bone ingrowth and fusion attachment with the adjacentvertebrae; said substrate block being relatively nonresorbable orresorbable at a rate substantially less than said surface coating.
 40. Aspinal fusion cage for implantation between and fusion with adjacentvertebrae, comprising: a substrate block including at least one loadbearing strut having high strength structural characteristics; and abio-active and resorbable surface coating carried by said at least onestrut, said surface coating having osteoconductive and osteoinductiveproperties to promote interbody bone ingrowth and fusion attachment withadjacent patient bone.
 41. The spinal fusion cage of claim 40 whereinsaid at least one strut substantially mimics the structuralcharacteristics of natural bone.
 42. The bone graft of claim 40 whereinsaid at least one strut is formed from a porous material.
 43. A spinalfusion cage for implantation between and fusion with adjacent vertebrae,comprising: a first region formed by load bearing anterior and posteriorwall connected by at least one load bearing strut; and a relativelyporous second region comprising the superior, inferior, and lateralaspects.
 44. The spinal fusion cage of claim 43 wherein said substrateblock where either the first or second portion comprises a ceramicstructure formed from silicon nitride, alumina, zirconia, zirconiatoughened alumina, hydroxyapatite, calcium phosphate, or compositionthereof.
 45. The spinal fusion cage of claim 43 wherein said substrateblock where either the first or second portion comprises a metallicstructure formed from titanium, tantalum, stainless steel, cobalt chromealloy, or composition thereof.
 46. The spinal fusion cage of claim 43wherein said substrate block where either the first or second portioncomprises a polymeric structure formed from peek, carbon fiberreinforced polymer, PMMA, PLA (or other bioresorbable polymer), orcomposition thereof.
 47. The spinal fusion cage of claim 43 wherein saidsubstrate block where either the first or second portion comprises aflexible material formed from silicone, polyurethane silicone,hydrogels, elastomers, or composition thereof.
 48. The spinal fusioncage of claim 43 wherein a bio-active and resorbable surface coatingapplied to said substrate block, said surface coating havingosteoconductive and osteoinductive properties to promote interbody boneingrowth and fusion attachment with the adjacent vertebrae.
 49. Thespinal fusion cage of claim 43 wherein said substrate block where thesecond portion is comprised of a bio-active and resorbable materialhaving relatively high osteoconductive and osteoinductive properties,such as a hydroxyapatite or a calcium phosphate material.
 50. The spinalfusion cage of claim 43 wherein said substrate block where the firstportion being relatively non-resorbable or resorbable at a ratesubstantially less than the second portion.
 51. The spinal fusion cageof claim 43 wherein the first region and the second region of the saidsubstrate block has a porosity ranging from about 0% to about 80% byvolume, and further wherein the pore size ranges from about 1 micron toabout 1,500 microns.
 52. The spinal fusion cage of claim 51 wherein thesaid first region of the said substrate block has porosity ranges fromabout 0% to about 50% by volume, and wherein the pore sizes range fromabout 1 micron to about 500 microns.
 53. The spinal fusion cage of claim51 wherein the said second region of the said substrate block hasporosity ranges from about 30% to about 80% by volume, and wherein thepore sizes range from about 100 microns to about 1000 microns.
 54. Thespinal fusion cage of claim 51 wherein the said substrate block has avariable porosity gradient substantially mimicking natural cortical andcancellous bone.
 55. The spinal fusion cage of claim 51 wherein saidsubstrate block has a first region of relatively low porositysubstantially mimicking natural cortical bone, and a second region ofrelatively high porosity substantially mimicking cancellous patientbone.
 56. The spinal fusion cage of claim 51 wherein said first regionhas a porosity of less than about 5%, and wherein said second region hasa porosity ranging from about 30% to about 80%.
 57. The spinal fusioncage of claim 51 wherein said first region is generally disposed on theexterior of said substrate block, and said second region is generallydisposed on the interior surfaces of said substrate block.
 58. Thespinal fusion cage of claim 51 wherein said second region is generallydisposed on the exterior of said substrate block, and said first regionis generally disposed on the interior surfaces of said substrate block.59. The spinal fusion cage of claim 55 wherein said first region isgenerally disposed on anterior and posterior surfaces of said substrateblock and further defines at least one structural load bearing strutextending through said substrate block between said anterior andposterior surfaces, said second region including an extended exposedsurface area for contacting the adjacent vertebrae.
 60. The spinalfusion cage of claim 59 wherein said second region is substantiallyexposed on medial and lateral surfaces of said substrate block.
 61. Thespinal fusion cage of claim 55 wherein said first regioncircumferentially surrounds and supports said second region, said secondregion including an extended exposed surface area for contacting theadjacent vertebrae.
 62. The spinal fusion cage of claim 55 wherein saidsecond region circumferentially surrounds said first region, said secondregion including an extended exposed surface area for contacting theadjacent vertebrae.
 63. The spinal fusion cage of claim 55 wherein saidfirst region comprises at least one structural load bearing strutextending through said substrate block, wherein said second regionincluding an extended exposed surface area for contacting the adjacentvertebrae.
 64. The spinal fusion cage of claim 43 wherein said substrateblock further includes means for facilitated grasping and manipulationwith a surgical instrument for implantation.
 65. The spinal fusion cageof claim 48 wherein said bio-active surface coating is internally andexternally applied to said substrate block.
 66. The spinal fusion cageof claim 48 wherein said bio-active surface coating is selected from thegroup consisting of hydroxyapatite and calcium compounds.
 67. The spinalfusion cage of claim 48 wherein said bio-active surface coatingcomprises a partially or fully amorphous osteoinductive materialincluding a glass and osteoinductive calcium compound.
 68. The spinalfusion cage of claim 48 wherein said bio-active surface coatingcomprises an organic coating material.
 69. The spinal fusion cage ofclaim 68 wherein said organic coating material is selected from thegroup consisting of autologous bone marrow aspirates, bone morphogenicproteins, growth factors and progenitor cells, and mixtures thereof. 70.The spinal fusion cage of claim 69 wherein said progenitor cells includemesenchymal stem cells, hematopoietic cells, and embryonic stem cells.71. The spinal fusion cage of claim 43 wherein the first region of thesaid substrate block is substantially radiolucent.
 72. The spinal fusioncage of claim 43 wherein the second region of the said substrate blockis substantially radiolucent.
 73. The spinal fusion cage of claim 43further including a therapeutic agent carried by said substrate block.74. The spinal fusion cage of claim 73 wherein said therapeutic agentcomprises a natural or synthetic osteoconductive or osteoinductiveagent.
 75. The spinal fusion cage of claim 43 wherein said substrateblock has a rough exterior surface.
 76. The spinal fusion cage of claim43 wherein said substrate block has a ribbed exterior surface.
 77. Thespinal fusion cage of claim 43 wherein said substrate block has alaterally open bore formed therein, and further including anosteoconductive material supported within said bore.
 78. The spinalfusion cage of claim 77 wherein said osteoconductive material comprisesmorselized bone graft material.
 79. The spinal fusion cage of claim 43wherein the pores formed within the second region of the said substrateblock are in substantially open fluid communication sufficient totransmit fluid pressure therebetween.
 80. The spinal fusion cage ofclaim 43 wherein the pores formed within the first region of the saidsubstrate block may be in substantially open fluid communicationsufficient to transmit fluid pressure therebetween.
 81. A spinal fusioncage for implantation between and fusion with adjacent vertebrae,comprising: a substrate block having a relatively high strengthcorresponding substantially with natural cortical and cancellous bone;and a bio-active and relatively rapidly resorbable surface coatingapplied to said substrate block, said surface coating havingosteoconductive and osteoinductive properties to promote interbody boneingrowth and fusion attachment with the adjacent vertebrae; saidsubstrate block being relatively nonresorbable or resorbable at a ratesubstantially less than said surface coating.
 82. A spinal fusion cagefor implantation between and fusion with adjacent vertebrae, comprising:a substrate block including at least one load bearing strut having highstrength structural characteristics; and a bio-active and resorbablesurface coating carried by said at least one strut, said surface coatinghaving osteoconductive and osteoinductive properties to promoteinterbody bone ingrowth and fusion attachment with adjacent patientbone.
 83. The spinal fusion cage of claim 82 wherein said at least onestrut substantially mimics the structural characteristics of naturalbone.
 84. The bone graft of claim 82 wherein said at least one strut isformed from a porous material.